1. Field of the Invention
This invention relates to titaniums having excellent impact resistance and its manufacturing method. Here, impact resistance is a property to stand impact applied from outside. Impact resistance is required of materials protecting human bodies or important products, wholly or partly, such as shields, helmets and bulletproof vests.
2. Description of the Prior Art
High-strength alloy steels and titanium alloys with high specific strength are used for products requiring high impact resistance, with a view to achieving weight savings. To attain this goal, warm or hot forming and low-speed forming have been employed.
Although the conventional processes described above improve formability, production efficiency is not high because processes to heat, keep the desired temperature, or descale after forming are involved. Besides, titanium alloys are more expensive than ordinary pure titaniums because vanadium, molybdenum or other alloying elements are added.
Although, in addition, titanium alloys have high strength, they do not have good impact resistance to high-speed impacts.
In order to eliminate the above shortcomings, an object of this invention is to provide pure titaniums having higher impact resistance than the conventional ones and a method for manufacturing such titaniums.
Another object of this invention is to provide pure titaniums with higher impact resistance and methods for manufacturing such titaniums at low cost.
Other objects of this invention are explicitly described in the following.
The studies made by the inventors to achieve the above objects led to a discovery that titaniums having excellent impact resistance can be obtained by controlling the quantities of oxygen, nitrogen and carbon contained in titaniums and applying work-hardening.
The titaniums according to this invention having an excellent impact resistance have a feature that the total content (S) of the contents of (O+N+C) is between 0.04 and 0.27 mass percent, the iron concentration is not greater than 0.1 mass percent, with the balance consisting of titanium and unavoidable impurities, and the Vickers hardness Hv* in the cross-sectional area satisfies one of the following equations (1), (2) and (3):
When 0.04xe2x89xa6Sxe2x89xa60.09 (mass percent)
150xe2x89xa6Hv*xe2x89xa6400xc3x97S+175xe2x80x83xe2x80x83(1)
When 0.09xe2x89xa6Sxe2x89xa60.20 (mass percent)
510xc3x97S+104xe2x89xa6Hv*xe2x89xa6400xc3x97S+175xe2x80x83xe2x80x83(2)
When 0.20xe2x89xa6Sxe2x89xa60.27 (mass percent)
510xc3x97S+104xe2x89xa6Hv*xe2x89xa6255xe2x80x83xe2x80x83(3)
xe2x80x83wherein S: [O]+[N]+[C] (mass percent)
Hv*: Vickers hardness in the cross-sectional area of the work-hardened product
A method for manufacturing the above titaniums according to this invention comprises applying preliminary working prior to forming so that the Vickers hardness Hv* in the cross-sectional area of the formed material satisfies one of the equations (1), (2) and (3) described above.
The titanium before the application of preliminary working may be in any condition; i.e., as hot-rolled or otherwise hot-worked, as cold-rolled or otherwise cold-worked, or annealed after such hot- or cold-working.
Another method for manufacturing the above titaniums according to this invention comprises applying, as said preliminary working prior to forming, rolling or leveling or both of them using rolls in a direction perpendicular to the direction of hot- or cold-rolling so that the Vickers hardness Hv* in the cross-sectional area of the formed material satisfies one of the equations (1), (2) and (3) described above.
Still another method for manufacturing the above titaniums according to this invention comprises applying annealing before or during forming so that the Vickers hardness Hv* in the cross-sectional area of the formed material satisfies one of the equations (1), (2) and (3) described above.
Oxygen, nitrogen and carbon are ordinary components of industrial pure titaniums contained usually in the range of 0.04 to 0.4, 0.01 to 0.02 and 0.001 to 0.02 mass percent respectively. This invention controls the contents of the oxygen, nitrogen and carbon not individually but in terms of the total content (S).
Here, preliminary working is a step indispensable to this invention that plays an important role in imparting the desired impact resistance to formed products. This step comprises cold rolling or leveling hot-rolled, hot- and cold-rolled, or hot- and cold-rolled and annealed sheets. The rate of cold reduction in preliminary working is not more than 70 percent or preferably between 10 and 50 percent. Leveling is applied using a tension leveler or a bending leveler.
Preliminary working is not limited to cold rolling and leveling. Forging and other method are also applicable. Although, a temperature near room temperature is preferable to preliminary working, with the prevention of oxidation in mind, there is no need to limit the temperature range.